In the field of microelectronic chip packages, the interface between the chip package and a cooling device is a dominant thermal resistance. Techniques to improve thermal conduction at this interface are important features in thermal management solutions, along with the traditional techniques of reducing the chip package internal and external thermal resistance. Thermal interfaces are typically formed by pressing a flat surface of the microprocessor chip and a flat surface of the cooling device together, with a thermal interface material (TIM) in between that is forced to flow into cavities or non-uniformities of the surfaces. A TIM is generally a particle-loaded viscous medium, which is also known in the art as a paste or grease or adhesive.
In U.S. Pat. No. 5,345,107, a cooling apparatus for an electronic device is disclosed that comprises a solid cooling body in close contact, through a TIM, with a heat transfer portion of an electronic device. The surface of the cooling device in contact with the electronic device has a number of grooves arranged to communicate with the outside of the heat transfer portion. A spring member is provided for elastically pressing the solid cooling body into close contact with the electronic device via the TIM in a third layer. The grooves are arranged in a perpendicular structure on the surface of the heat transfer portion and the capacity of the grooves is arranged to be larger than the volume of the TIM surrounding the grooves and between the two heat transfer surfaces. This construction allows the solid cooling body to be brought into close contact with the electronic device by means of the TIM.
U.S. Patent Publication number 2005/061474 discloses a thermal interface for chip cooling. A flexible, thermally conductive enclosure containing a TIM of thermally conductive liquid or paste-like metal(s) is described. The enclosure is adapted to be placed between the flat surfaces of a chip and a heat sink to enhance heat transfer from the chip to the heat sink. The thermal interface enclosure is held in place by mechanical pressure rather than by bonding, thereby facilitating inspection and repair of the microprocessor device.
Generally, a TIM comprises a viscous matrix and a load of suspended conductive particles, which particles can be on a micro or nano scale. It is widely assumed that use of a low viscosity matrix, and thus a TIM of low effective viscosity, is preferable because such a material has a higher squeeze flow velocity (i.e., it can be distributed between the nearly adjacent surfaces more quickly). See, for example, “Thermal Resistance of Particle Laden Polymeric Thermal Interface Materials” by R. Prasher, J. Shipley, S. Prstic, P. Koning, J. L. Wang, in ASME Journal of Heat Transfer, vol. 125, December 2003. Thus, while it is preferable, in theory, to have a relatively large load of conductive particles in the TIM in order to increase thermal conductivity, in practice, larger loads have been shown to lead to an increased number of particle interactions and increased effective viscosity.
Thus, it is known in the art to position the microprocessor chip and the cooling device close together with a low effective viscosity TIM between them as a preferred way to increase the thermal conductivity to ambient and to improve microprocessor cooling.
Several problems are associated with known thermal interfaces. First, it is found that there is an unexpected limit on the proximity of two surfaces when pressed together in a thermal interface. For example, this limit is evident in results of experimental data for a TIM having a relatively low matrix viscosity of 100 cSt. It can be seen that, when using a 100 cSt viscosity TIM in a thermal interface having two flat surfaces, interface gap thickness increases markedly when the load of particles in the TIM exceeds a certain threshold, here, approximately 55% by volume. The minimum achievable distance between the surfaces is typically more than ten times the diameter of the average TIM particles, which corresponds to a substantial decrease in thermal conductivity.
Second, use of a TIM of low effective viscosity, due to a relatively low load of particles, entails the following problem. Experimental data shows that when the load of particles falls below a certain threshold, the effective conductivity is relatively close to the low value of the matrix material. The drop of conductivity is due to the fact that below the threshold there is little interaction between particles and the heat must pass through the low conductivity matrix. Above the loading threshold, heat is conducted by a “percolation” mechanism whereby random arrangements of interacting particles create a preferred thermal path. The effective conductivity above the percolation threshold is, however, still limited to a relatively low value compared to the expected conductivity of the particles mixed with the matrix.
Third, non-uniform thermal properties are observed at known interfaces, which results in significant temperature gradients within the chip. In particular, relatively smaller areas of high thermal conductivity and relatively larger areas of reduced conductivity are observed. When there is a relatively large distance between the two regions compared to the thickness of the chip or cooling device, the temperature profile along the surface develops into similar regions of maximum and minimum temperatures. Maximum chip temperature is a parameter that determines thermal reliability of a chip and, therefore, temperature gradients are detrimental.
Fourth, when the chip package is thermally cycled, due to on-off cycles or varying processing loads, an interface warping between hot and cold positions is observed due to differences in thermal expansion coefficients between the components in the system. This warping not only causes an undesirable TIM pumping effect which increases with a low effective viscosity TIM, but is also detrimental to the long term reliability of the interface due to voiding.
It is an aim of the present invention to provide a thermal interface which mitigates the problems of the known art. It is a further aim to provide a technique for the controlled placement of particles on various scales.